An electrophotographic image forming apparatus includes a developer device having a developer material bearing member which brings developer material, such as toner particles, onto an electrostatic latent image bearing member for development and a toner supply roller which is disposed in contact with the developer material bearing member and supplies toner particles to and collects them from the developer material bearing member at the contact area thereof.
The United States Patent Application No. 2001/0036376 A1 discloses such toner supply roller which includes a core bar and a form layer disposed around the circumference of the core bar. The circumferential layer is made of resin foam, such as urethane foam, or rubber foam, which can cause certain disadvantages due to the property of the material. For example, a foam layer made of a highly permeable material has an inferior ability of scraping off toner particles from the developer material bearing member, which results in that the toner particles on the developer material bearing member can not be replaced with fresh toner particles and thereby leads a degradation of toner particles. The degraded toner particles can only weakly adhere to the developer material bearing member due to the decrease in electric charge, which causes the toner particles to drop off from the developer device. A foam layer made of a material with low density will be pressed softly against a developer material bearing member, and therefore, can exercise less scraping-off ability. This may degrade toner particles on the developer material bearing member and will therefore tend to result in a possible dropping of the toner particles.
A foam layer made of a material of higher density, on the other hand, can be pressed against the developer material bearing member with a high pressure, which results in that a relative slide between the foam layer and the developer material bearing member can cause an invasion or implantation of the external additive into the surfaces of toner particles. This in turn deteriorates the functions of the external additive for providing a fluidity for toner particles and controlling electric charge therefor, for example.
Further, a foam layer made of a material with less hysteresis loss rate, i.e., a ratio of mechanical energy loss per deformation/recovery cycle, can readily recover from its deformation caused by the contact with the developer material bearing member and will therefore exercise a stable adhering to the developer material bearing member. This may result in a relative slide between the foam layer and the developer material bearing member and thereby the degradation and the resultant dropping of toner particles.
A foam layer made of a material with a higher hysteresis loss rate, on the contrary, exercises a less ability of adhering to the developer material bearing member and a less ability of toner scraping. This in turn results in a rapid degradation and a frequent dropping of the toner particles.
As described above, such inconvenience will occur unless the material of a foam layer of a toner supply roller has a proper air permeability, a proper density and a proper hysteresis loss rate.